DbgValueHistoryCalculator: Store modified registers in a BitVector instead of std::set.

And iterate over the smaller map instead of the larger set first.  Reduces the time spent in
calculateDbgValueHistory by 30-40%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219123 91177308-0d34-0410-b5e6-96231b3b80d8